Population Ecology & Exponential Growth

Sudden unavailability of a required resource, such as food, can cause the up-and-down shape of exponential growth (see References 2, page 183).

Using an exponential form of population growth, two fruit flies could produce enough offspring to fill the space between the Earth and the Sun in only one year, if all of the offspring survived (see References 1). While exponential growth is one of two growth rates ecologists recognize in nature, other factors keep the world from becoming overrun with fruit flies or other organisms that show exponential growth.

Exponential Growth Form

Ecologists also call exponential growth forms "J-shaped forms" because, if you graph the number of organisms over a period of time, the resulting graph looks like the letter "J." Gradual population growth continues over a relatively long period of time and suddenly increases dramatically, producing an almost vertical line on the graph. To calculate exponential growth, multiply the growth rate times the number of organisms to obtain the number of offspring produced per generation. Microbes offer a dramatic example. If a bacterium divides itself every 24 hours, a single organism becomes two bacteria after one day. By the second day, there are four bacteria. After only a month, populations have skyrocketed past 1 billion. (See References 2, page 182)

Environmental Limitations

Environmental limitations prevent fruit flies or microbes from overtaking the planet. Earth supplies finite resources to support a population, and once organisms outnumber resources, populations tend to drop off as individuals compete for limited resources. For example, if a population of deer consumes all of its food sources, populations drop before beginning to rise again as food sources regenerate. On a graph, this cycle produces a series of rapid population spikes followed by equally dramatic declines. (See References 2, page 184)

Survival Rates

Scenarios like those with the fruit flies or microbes sound dramatic but do not represent typical population growth. In nature, some organisms die as others are born or generated. In particular, organisms that produce large numbers of offspring tend to have very few of those offspring survive to reproductive age themselves, severely curbing that species' population growth. (See References 1)

Logistic Growth and Carrying Capacity

Natural populations don't tend to demonstrate the dramatic ups and downs of exponential population growth. Instead, as populations reach resource limitations, growth slows and populations stabilize, a growth form called logistic (see References 1), sigmoid or S-shaped (see References 2, page 182). The term "carrying capacity" describes the number of individuals an ecosystem can support (see References 1). For example, plants require nitrogen from the soil to grow, so nitrogen levels can limit the number of plants that can grow on a plot. After seeding a bare plot of land with dandelion seeds, the population of dandelions increases quickly. As the population begins to use up the nitrogen in the soil, it reaches carrying capacity and population growth slows. Soon, the plot of land contains the number of dandelions the nitrogen in the soil can support, reflecting the carrying capacity.